Steel substrate for reinforcement of elastomers

ABSTRACT

The invention relates to a substrate for reinforcing elastomeric polymers whereby at least part of the substrate is made of steel, said part being covered by a layer of an alloy consisting of, apart from impurities, between 4.2 and 6.5% weight of aluminum, possibly less than 0.1% of at least one element stimulating the wetting ability of the liquid alloy to the substrate and the balance zinc.

BACKGROUND OF THE INVENTION

The present invention relates to a substrate for reinforcement ofelastomeric polymers wherein at least part of the substrate is made ofsteel. Steel wires and cords comprising steel wires twisted together(possibly together with other synthetic filaments such as aramid fibers)are often used for reinforcing rubber products such as tires, belts andhoses. In view of securing a proper and durable adhesion to the rubber,the wire surfaces are generally coated with an alloy layer such as brassor zinc.

Besides a proper adhesion capacity, the coating layer should preferablyalso protect the wires against corrosion attack. Indeed, corrosion ofthe reinforcing steel structure should always be avoided as thereinforcing effect decreases as a consequence of corrosion. Besidesexposure of the steel elements to atmospheric corrosion before theirembedment into rubber, corrosion attack is also possible after suchembedment, especially when incisions in the rubber, which reach the wiresurfaces, are produced.

Numerous efforts have been made up to now to design specific coatinglayers for steel wires which offer a good adhesion capacity (also afterageing of the reinforced composite) in combination with a propercorrosion resistance. Unfortunately, the application of those coatinglayers requires quite complicated processes which generally raise theproduction cost of the coated reinforcing material. Further, the coatingprocess often becomes quite critical when steel wires are involved withelevated tensile strength e.g. over 3000 N/mm², as those wires oftenrequire specific manufacturing processes.

SUMMARY OF THE INVENTION

It is now a primary object of the invention to provide a relativelysimple coating composition and process for a reinforcing steel substratewhich offers adequate adhesion strength (and adhesion retention afterageing) to the surrounding elastomeric matrix combined with an improvedresistance against static and dynamic corrosion attack. In particular itis an object of the invention to provide a reinforcing substrate forelastomeric polymers as defined in claim 1.

It is a second object of the invention to provide such coatings on steelwire substrates with an elevated tensile strength.

According to another object of the invention, a bundle, e.g. a twistedcord or cable is provided comprising a number of said steel wires,possibly combined with filaments of other material.

Another object of the invention deals with the combination of steelwires of different kinds in said bundle or cord, e.g. wires withdifferent diameter and/or strength.

Yet another object of the invention relates to the combination of thesimple coating composition and/or process with the deposition of aspecific sublayer and/or top layer of another material in view ofmeeting specific requirements for adhesion and/or corrosion resistance.

A further object of the invention resides in methods and means formanufacturing and using said steel substrates ,and said combinations ofsubstrates.

An additional object of the invention concerns the elastomeric productsreinforced with said substrates such as conveyor belts, transmissionbelts, (high pressure) hoses, tires etc.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the invention, the relatively simple coating layercomposition for the reinforcing substrate comprises an alloy which,apart from impurities, consists of between 4.5 and 6.5wt. % ofaluminium, possibly less than 0.1% of at least one element stimulatingthe wetting ability of the liquid alloy to the substrate and the balancezinc. At least a part of the substrate is made of steel, and the abovecoating layer composition is applied to at least some portions of saidpart.

It is known from the Japanese patent application 59-173257 to coat awire with a Zn alloy including 2.5 to 7% Al.

The weight of said layer according to the invention is between 10 and 60g per m² of the covered surface of the substrate. Steel wire is asuitable reinforcing substrate. The steel thereby has a carbon contentof at least 0.4 wt. % and preferably between 0.7 and 1 wt. %. Further,the steel wire has a tensile strength Rm of at least 2100 N/mm². Howeverwires with a tensile strength of at least 3100 N/mm² are alsocontemplated. In particular wires with Rm≧2250-1130 log d are envisagedwherein d is the diameter of the wire. The wire may have a round, squareor rectangular cross section.

The reinforcing substrate according to the invention can consist of anumber of single wires, however it can also comprise a number offilaments bundled together wherein at least one of the filaments is asteel wire with a diameter between 0.08 mm and 0.50 mm. The filamentsare preferably bundled together by twisting. Steel wires can then bedisposed either in the center of the bundle, in the circumference and/orin an intermediate layer between core and outer layer of the bundle. Ifdesirable, only part of the filaments in either core, circumferential orintermediate layer may be of steel. Often however, all filaments in thetwisted bundle will be steel wires.

Further, not all wires in the twisted substrate should have the samediameter or the same tensile strength. A number of wires can have adiameter and/or tensile strength which is different from the diameter orstrength of any other wire or filament in the twisted bundle. Inparticular, a number of wires can have a tensile strength Rm>2250-1130log d.

In cases where adhesion and adhesion retention is required to specificrubber compounds, it may be desirable to further cover the steel wire,already provided with the Zn/Al-alloy layer according to the invention,with an additional layer promoting said adhesion to the specificelastomeric polymers. The additional layer may be a metal layercomprising Cu, Zn, Ni and/or Co. In particular said metal layer maycomprise brass.

In other instances it may be contemplated to deposit an intermediate orsubcoating on the wire substrate before applying the Zn/Al-alloy coatingaccording to the invention. Such a subcoating may comprise Zn and/or Ni.

The invention covers also elastomeric products, reinforced withsubstrates having the specific Zn/Al-alloy-coating layer at theirsurface. Hose reinforcement steel wires, hose wire cords, respectivelyconveyor belt cord with said Zn/Al-alloy coating as well as the soreinforced hoses, particularly high pressure hoses, resp. conveyor anddriving or transmission belts are contemplated.

EXAMPLE 1

A steel cord according to the invention (specimen 2 in the table below)and for the reinforcement of a rubber conveyor belt was prepared withthe following characteristics: the cord comprised 7 strands twistedtogether. Each strand consisted of 7 steel wires twisted together. Eachwire had a diameter of 0.42 mm, a carbon content of 0.86 wt. % and aZn--Al-alloy layer with a weight of 42 g per m² of wire surface. TheZn--Al-alloy comprised about 5 wt. % of Al and about 0.02% La and about0.02% of Ce as a wetting agent to steel. Besides other impurities thebalance of Zn amounted to about 95 wt. %.

The same cord (7×7×0.42--specimen 1--) was prepared; however each wirehad a coating of zinc (hot dip) of about 50 g per m² of wire surface. Asexplained above, the eutetic Zn--Al-coating has an excellent corrosionresistance which is generally at least three times the corrosionresistance of conventionally galvanised (hot dip Zn-coated) wire whensubmitted to a salt spray test. This is the reason why corrosion testswere not repeated here.

Applicant however had very much doubts as to the adhesion capacity andadhesion retention after aging of the new Zn--Al-coatings, when comparedto Zn-coatings. Therefor the Zn--Al-coated cords described above wereembedded and vulcanised in two rubber compounds for conveyor belts. Thepull-out force (N/mm) was determined as per AISI/ASTM test. No. 2630 aswell as the appearance rating (APR) which is a visual estimation of thedegree of rubber coverage after peeling the rubber from the cord layer.

The table 1 below represents the values obtained for each of twocompounds A and B, for the Zn-coated cord (specimen 1) and for theZn--Al-coated cord (specimen 2).

                  TABLE 1                                                         ______________________________________                                                          aged       aged                                                     initial   adhesion   adhesion                                                 adhesion  180° C. - 90'                                                                     150° C. - 240'                                    N/mm   APR    N/mm    APR  N/mm   APR                                 ______________________________________                                        specimen 1                                                                    (state of the art)                                                            Comp. A   134      7.0    104.7 9    108.3  8.3                               Comp. B   131.7    7.0    --    --   137.0  8.7                               specimen 2                                                                    (invention)                                                                   Comp. A   135.0    8.0    101.7 9.0  121.3  9.0                               Comp. B   119.3    8.0    --    --   148.0  8.3                               ______________________________________                                    

The results obtained indicate that values for initial adhesion (freshlyvulcanised composite rubber/cord) are quite comparable for bothspecimens. This means that the adhesion capacity for Zn--Al-coated cordsaccording to the invention is generally not worse than forconventionally Zn-coated cords. Surprisingly however, the adhesionretention after aging is also excellent for the cords according to theinvention and overall even slightly better than for conventionallyZn-coated steel cords. From the above data can thus be concluded thatthe Zn/Al-coated substrates according to the invention offer at the sametime a better corrosion resistance and an adhesion strength to rubberwhich is in general at least equal to that of conventionally Zn-coatedsubstrates, even after aging. The better corrosion resistance does notonly relate to circumstances of static corrosion but also to those ofdynamic corrosion which then results in a better corrosion fatigueresistance.

As a proof thereof wet and dry fatigue tests were carried out as set outin example 2 below.

EXAMPLE 2

Steel wire filaments with substantial residual compressive stresses attheir surface were coated with the Zn/Al-alloy coating described inexample 1. They had a diameter of 0.19 mm resp. 0.21 mm and a tensilestrength of between 3600 and 3850 N/mm² resp. between 3400 and 3600N/mm². Three different coating amounts were present on the filaments.The heaviest coating had a weight of about 35 g/m² of filament surfacewhereas the coating with the lowest weight was about 11 g/m². Anintermediate coating amount of about 25 g/m² was tested also.

Conventional fatigue tests were carried out (540.000 cycles) in dry (35%relative humidity) and wet (demineralized water) conditions as describede.g. at the bottom of page 4 of the published European patentapplication No. 220.766. The results are summarized in the table 2below:

                  TABLE 2                                                         ______________________________________                                                coating               corrosion fatigue                               diameter                                                                              weight   dry fatigue limit                                                                          limit (wet)                                     (mm)    g/m.sup.2                                                                              N/mm.sup.2   N/mm.sup.2                                      ______________________________________                                        0.19    33       1300         1200                                                    22       1400         1100                                                    13       1500          925                                            0.21    37       1000          975                                                    36       1300         1025                                                    11       1350         1000                                            ______________________________________                                    

Professionals in the field will certainly recognise that the values intable 2 are very high.

EXAMPLE 3

A tire cord was prepared of the construction 3×0.21+9×0.19 with a cablepitch of 12.5 mm. The filaments (used in example 1) with a diameter of0.19 mm and with the Zn/Al-alloy coating weight of 13 g/m2 were unwoundfrom the cord and submitted to the same corrosion fatigue test (wetconditions) as described in example 2. The corrosion fatigue limit valuewas about 825 N/mm2 which is still considered satisfactory. In fact, dueto the twisting operation, corrosion fatigue limits decreased from 925N/mm2 (example 2) only by about 10 %. The filaments with a diameter of0.21 mm had a Zn/Al-alloy coating weight of 11 g/m2.

EXAMPLE 4

The cords (1) according to the invention and described in example 3 wereembedded in a rubber compound comprising as quantitatively mostimportant ingredients per 100 parts of rubber: 45 parts of C. B. Regal300; 12.5 parts of Ultrasil VN 3; 8 parts of ZnO; 6 parts of Dutrex 729;6 parts of sulfur; 5 parts of Cofill 11; 4 parts of Cyrez 963; 2 partsof Santoflex 13 and 1.5 parts of Manobond C 16. The composite wasvulcanised for about 25 min. at 150° C.

Adhesion (expressed in N) was determined according to the conventionalpull-out test and the appearance rating (APR in %) was noted. The sametests were carried out for comparison on similar cords (2), (3), (4)(same construction and similar tensile strengths). Cords (2) had on topof the Zn/Al-alloy coating a very thin Co-coating (1000 nm) applied byphysical vapor deposition. Cords (3) were conventional brass coatedcords (about 63% Cu and 37% Zn) and cords (4) were the same brass coatedcords with again a thin Co-layer (of about 1000 nm in thickness) appliedby physical vapor deposition. Table 3 summarises the results. Adhesionis somewhat lower for the cords (1) and (2) compared to the brass coatedcords (3) and (4) but much better than normally would have been expectedby persons skilled in the art. The influence of Co is not verysignificant for the rubber compound used in these experiments.

                  TABLE 3                                                         ______________________________________                                        cord           adhesion APR                                                   type           (N)      (%)                                                   ______________________________________                                        (1)            500      89                                                    (2)            514      90                                                    (3)            578      91                                                    (4)            568      93                                                    ______________________________________                                    

We claim:
 1. A reinforcing substrate with improved adhesion retention toelastomeric polymers comprising a plurality of filaments wherein atleast one of said filaments is a steel wire covered at least in part bya layer of an alloy consisting of, apart from impurities, between 4.2and 6.5 wt. % of aluminum, a wetting element which is present in anamount less than
 0. 1% sufficient to stimulate the wetting ability ofthe alloy when liquid to the substrate, and the balance zinc.
 2. Asubstrate element according to claim 1, wherein the weight of said layeris between 10 and 60 g per m² of the covered surface of the substrate.3. A substrate according to claim 1 wherein said steel wire has a carboncontent of at least 0.4 wt. %.
 4. A substrate according to claim 3wherein said steel wire has a carbon content between 0.7 and 1 wt. % ofcarbon.
 5. A substrate according to claim 3 wherein the steel wire has atensile strength Rm of at least 2100 N/mm².
 6. A substrate according toclaim 5 wherein the steel wire has a tensile strength of at least 3100N/mm².
 7. A substrate according to claim 5 wherein the tensile strengthRm is larger than 2250-1130 log d wherein d is the diameter of the wire.8. A substrate according to claim 1 wherein the steel wire has arectangular cross-section.
 9. A substrate according to claim 1comprising a number of filaments bundled together, wherein at least oneof the filaments is a steel wire with a diameter between 0.08 mm and0.50 mm.
 10. A substrate according to claim 9 wherein the filaments arebundled together by twisting.
 11. A substrate according to claim 10wherein at least a part of the filaments in the twisted bundle arecentrally disposed filaments which are steel wires.
 12. A substrateaccording to claim 10, wherein at least a part of the filaments in thetwisted bundle are circumferentially disposed filaments which are steelwires.
 13. A substrate according to claim 10 wherein the twisted bundleincludes centrally and circumferentially disposed filaments, and whereinat least a part of those filaments disposed between the centrally andcircumferentially disposed filaments are steel wires.
 14. A substrateaccording to claim 10 wherein all the filaments are steel wires.
 15. Asubstrate according to claim 10 or 14 wherein a number of the wires havea diameter which is different from the diameter of any other wire orfilament in the twisted bundle.
 16. A substrate according to claim 10 or14 wherein a number of the wires have a tensile strength which isdifferent from the tensile of any other wire or filament in the twistedbundle.
 17. A substrate according to claim 16, wherein the wire has adiameter d, wherein said number of wires have a tensile strengthRm>2250-1130 log d.
 18. A substrate according to claim 1 wherein saidalloy layer is covered at least in part with another layer promoting theadhesion to elastomeric polymers.
 19. A substrate according to claim 18wherein said other layer comprises Cu, Zn, Ni and/or Co.
 20. A substrateaccording to claim 19 wherein said other layer comprises brass.
 21. Asubstrate according to claim 1 or 18 wherein said alloy layer isdeposited on an intermediate layer comprising Zn and/or Ni.
 22. The useof a steel substrate for the reinforcement of products comprisingelastomeric polymers, wherein said steel substrate is covered at leastin part by a layer of an alloy consisting of, apart from impurities,between 4.2 and 6.5 wt. % of aluminum, a wetting element which ispresent in an amount less than 0.1% sufficient to stimulate the wettingability of the alloy when liquid to the substrate, and the balance zinc.23. An elastomeric polymer product reinforced with a substrate accordingto claims 1 or
 22. 24. An elastomeric polymer product according to claim23 in the form of a conveyor belt.